New docs + better node breakers

The docs now talk about node breakers and deployers, plus are updated according to the latest commits changing the tube connection behavior. Oh, and they have a nice new background as well.

The other commit improves node breakers so they can't just go around destroying nodes willy-nilly. It implements all the functionality of `minetest.node_dig`, basically, including all the callbacks and checks. This means you can dig active mesecons and it'll update the circuit, and you can't dig locked chests or chests with any items in them. Basically, it plays better with everything else.

Note that I haven't tested the new node breaker behavior very much, but a few basic tests seem to work fine.